1. Field of the Invention
The present invention relates to a solid-state image sensor, and more particularly, it relates to a solid-state image sensor comprising gate electrodes transferring charge.
2. Description of the Background Art
A solid-state image sensor comprising gate electrodes transferring charge is known in general, as disclosed in Japanese Patent Laying-Open No. 2001-53263, for example.
FIG. 10 is a sectional view for illustrating the structure of an exemplary conventional solid-state image sensor having a structure similar to that of the solid-state image sensor disclosed in the aforementioned Japanese Patent Laying-Open No. 2001-53263. Referring to FIG. 10, a plurality of pixels 101 are adjacently arranged in the exemplary conventional solid-state image sensor. This exemplary conventional solid-state image sensor comprises an n-type substrate 102. A p-type well region 103 is formed on a region of a prescribed depth from the upper surface of the n-type substrate 102. A plurality of n-type high-concentration impurity regions 104 are formed on prescribed regions of the surface of the n-type substrate 102 at prescribed intervals. The n-type high-concentration impurity regions 104 are formed on regions shallower than the p-type well region 103.
P-type impurity regions 105 are formed on the surfaces of the n-type high-concentration impurity regions 104. These p-type impurity regions 105, the aforementioned n-type high-concentration impurity regions 104 and the p-type well region 103 constitute photoelectric conversion portions 106. These photoelectric conversion portions 106 are provided in one-to-one correspondence to the pixels 101 respectively. N-type transfer channel regions 107 are formed on regions of the surface of the n-type substrate 102 located between adjacent pairs of photoelectric conversion portions 106 respectively. These transfer channel regions 107 function as paths for transferring charge generated in the photoelectric conversion portions 106.
A gate insulating film 108 is formed to cover the overall surface of the n-type substrate 102. Gate electrodes 109a and 109b transferring charge are formed on regions of the gate insulating film 108 corresponding to the transfer channel regions 107. Each pair of gate electrodes 109a and 109b are provided in one-to-one correspondence to each pixel 101. The gate electrodes 109a and 109b included in each pixel 101 are arranged at a prescribed space, to hold the corresponding photoelectric conversion portion 106 therebetween. The gate electrode 109a of a prescribed pixel 101 and the gate electrode 109b of another pixel 101 adjacent to the prescribed pixel 101 are arranged through an insulating film 110. The gate electrode 109b of the prescribed pixel 101 partially overlaps on the gate electrode 109a of the adjacent pixel 101.
In imaging, a low-level voltage is applied to the gate electrodes 109a and 109b, so that the potentials of the transfer channel regions 107 located under the gate electrodes 109a and 109b are higher than those of the photoelectric conversion portions 106. Thus, potential wells are formed in the photoelectric conversion portions 106 for storing charge generated by photoelectric conversion in imaging. At this time, potential barriers are formed on the transfer channel regions 107 for separating the potential wells formed in pairs of photoelectric conversion portions 106 adjacent to each other through the transfer channel regions 107.
In the exemplary conventional solid-state image sensor shown in FIG. 10, however, the potentials of the transfer channel regions 107 are disadvantageously reduced when n-type impurity concentrations in the transfer channel regions 107 are increased in order to increase the quantity of charge transfer. Thus, the height of the potential barriers formed on the transfer channel regions 107 in imaging is so reduced that the charge may flow out from the potential well of a prescribed photoelectric conversion portion 106 into the potential well of another photoelectric conversion portion 106 adjacent to the prescribed photoelectric conversion portion 106 through the corresponding transfer channel region 107. This outflow of the charge disadvantageously results in blooming, a phenomenon brightly displaying unirradiated portions. When the n-type impurity concentrations in the transfer channel regions 107 are increased, further, hole concentrations are disadvantageously reduced in the transfer channel regions 107. If unintentional charge is resulting from thermal excitation or the like in the transfer channel regions 107 located under OFF-state gate electrodes 109a and 109b in this case, the quantity of unintentional charge not captured by holes is disadvantageously increased. Thus, a dark current resulting from such unintentional charge is disadvantageously increased.